Tuesday, December 21, 2010

Shadow cones

Today we had a total lunar eclipse. In theory it could have been a wonderful sight. In practice it was unspectacular because:

The moon was near the horizon at the start of the eclipse and it was setting. So most of the sight would have been out of view anyway.

It was very cloudy.

When I looked up the coordinates in Stellarium and pointed my camera in the approximate direction this is what I saw. I think the moon was somewhere above the middle street light.

A lunar eclipse occurs when the moon enters the shadow cone that is cast by the earth. This cone has two parts: the umbra and the penumbra. The penumbra is the bigger, widening shadow cone emanating from the earth.

I could not see this effect on the astronomical scale, but I was reminded of a recent "discovery" that I made. The discovery of "urban shadow cones".

A few weeks ago I looked at a street light at just the right angle and I noticed a beautiful, big, pyramidal shadow cone. Somehow I had never noticed this phenomenon before. I had to trudge through the snow a few times before I found a good example that produced a nice clear photograph.

It took some careful observing, remembering and web-searching before I found and verified the answer: new streetlights! During 2010 the city of Rotterdam has been replacing the old sodium lamps with new metal-halogen lamps. These behave more like a point-source and produce less diffuse light - and thus produce much sharper and clearer shadows. I had not noticed these before because they did not exist yet.

It is easy to prove with a picture of the old type of street light - they are still there in the smaller side streets. Notice the combination of umbra and penumbra in this example. But here it is produced by two light sources at different distances from the lamppost that casts the shadows. So the geometry is different from the astronomical case. And just for completeness: an omnidirectional light source casts no shadow at all.

2 comments:

Sorry, but you are wrong in this: "And just for completeness: an omnidirectional light source casts no shadow at all."

Just imagine the sun, humankinds most well known light source.. Try the wunderful structures cast by the intensive omnidirectional light of a point source in the form of a small halogen bulb.What you get produced by that bulb are very sharp & defined shadows.

The difference to the street light is, that the latter is diffuse - so produces diffuse shadows.

You are right of course.The word "omnidirectional" is not correct. It usually means "something that radiates in all directions".What I think I meant was: An object that is surrounded by light from all directions (the lamppost) will produce no visible shadow.But you are right, there is a shadow but it is uniformly distributed, and thus invisible :-)